FIG. 1 shows a ducted fan gas turbine engine 10 comprising, in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. The fan, compressors and turbine are all rotatable about a principal axis 31 of the engine 10. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by interconnecting shafts 38, 40, 42.
The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is generally improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbines at the highest possible temperatures. As a result, the turbines in state of the art engines, particularly high pressure turbines, operate at temperatures which are greater than the melting point of the material of the blades and vanes making some form cooling necessary.
Typically, components are cooled by a flow of compressed air which is at a higher pressure than the main gas path but a significantly lower temperature. Components are provided with internal cooling passages which both distribute the cooling air and act to internally cool a particular component.
A continuing challenge of providing cooling passages within components is to improve the tolerance with which the passages can be placed within components so that the wall thickness of a component can be reduced so far as possible.
Typically, cooling passages can be provided by so-called lost wax method or investment casting of components as is well known in the art of casting technology. Lost wax casting involves the principal steps of forming a ceramic core, surrounding the core with a wax (or other suitable sacrificial material), prior to coating the waxed core with a ceramic shell. The core defines an internal cavity within the cast metal component, the wax defines the space in which metal will be cast, and the shell defines the external surface of the cast metal component.
The core may be injection moulded prior consolidation by drying and optionally firing. The core is then placed in a second mould and wax is injected. The wax covered core is then repeatedly dipped in ceramic slurry to provide the shell. Once the shell is dry, the wax is removed using the appropriate process as defined by the chemistry of the wax (e.g. by soaking in water for a water soluble wax, or heating) and the vacated mould fired to ready it for receiving molten metal. To cast the object, metal is poured into the cavity which has been provided by the removed wax. After the metal has solidified, the ceramic parts are removed by a leaching process to leave the cast metal component which may be further processed by machining or annealing for example.
Known problems with ceramic cores is the inevitable shrinkage and warping during the drying an firing thereof, and the wax encapsulation which may involve a high pressure injection with resultant mechanical stresses on the core parts. Thus, in any core production there will be a manufacturing tolerance which must be accommodated.
One effect of providing this tolerance is the addition of material to the walls of the cast component so as to guarantee a minimum wall thickness after any movement or shrinkage is allowed for. However, providing a minimum wall thickness may be problematic where the wall thickness needs to be as low as possible, for example, to reduce the component weight or allowing the performance of the resultant cast component as predictable as possible.
The straying of core sections away from an expected or desired position is more notable for longer core passages in which there is an accumulation of error along the length of the passage and the elongate geometry results in an inherently more flexible structure which is less able to withstand the wax injection or subsequent processing steps without drifting from the required position.
The movement of sections of a core is most notable when a relatively long core section is tortuous such that the passage length between two points is significantly greater than the direct separation between the two points. Thus, the movement accumulated over the length is presented across a smaller separation.
One way to combat relative movement between core passages is to use so-called core ties which extend between adjacent core passages and provide some stability. These core ties may be ceramic, and thus form part of the cooling passage once the ceramic has been removed. This leads to the addition of a potentially unwanted cooling path joining adjacent passages which short circuits some of cooling circuit.
Another method of providing core stability is to use metallic core ties which are subsumed into the cast metal part due to the relative melting point of the ties and liquid metal used to cast the part.
Both of these methods are suitable for particular core passage geometries, but are lacking for others. The present invention seeks to provide an improved method of tying core passages together.
This invention seeks to provide an improved core structure and method of casting a component which allows for more accurate placement of the cooling passages to allow for improved components with more predictable cooling properties and the potential for reducing the wall thickness of component.